Sensing & Responding

Movement Above Ground
Although plants cannot walk, throughout their lives they seek, search, and avoid. Like other organisms, plants move, orienting themselves to access resources or to avoid injury. This concept is easily appreciated in animals but is confusing in plants. For example, a search for a pencil in the dark relies on sensors in our fingers, but control of the muscles and the decision to grasp are located in our human brains. From where in a plant does control for the adjustment of stem or root originate? Can all plant movements be explained as responses to physical stimuli?

Sun Dance, Roger Hangarter, time-lapse movie,
sunflower seedlings (Helianthus annuus).

Sun Worship shows a time-lapse sequence that illustrates plant responses to light and gravity. When the light from the bulb is on, the surrounding corn seedlings appear to genuflect, as if in worship of the light. But there is a less romantic interpretation. The seedlings’ behavior shows a conflict between their attraction towards light (phototropism) and their tendency to grow upright in response to gravity (gravitropism).

Sun Dance reveals sunflower seedlings as they grow and “reach” for the light, wriggling wildly as if in celebration of life. But such “festivity” is not confined to sunflowers; most plants exhibit this motion to some degree. When seedlings emerge from the soil, they search for the orientation that provides the greatest sunlight. To remain upright as they grow, they correct for the “leaning” that results from being temporarily imbalanced in the direction of light. The wobbling, circular, swaying movement (called nutation) facilitates exploration of their immediate environment.